The present invention relates generally to lighting devices and more particularly to an organic light emitting diode.
Organic electroluminescent devices, such as organic light emitting diodes (OLEDs), are currently used for display applications and are planned for use in general lighting applications. An OLED device includes one or more organic light emitting layers disposed between two electrodes, e.g., a cathode and a light transmissive anode, formed on a light transmissive substrate. The organic light emitting layer emits light upon application of a voltage across the anode and cathode. Upon the application of a voltage from a voltage source, electrons are directly injected into the organic layer from the cathode, and holes are directly injected into the organic layer from the anode. The electrons and the holes travel through the organic layer until they recombine to form excited molecules or excitons. The excited molecules or excitons emit light when they decay.
However, the external quantum efficiency of OLEDs, which is defined as a ratio of the photons emitted by the device to the number of injected electrons is lower than desired. There have been prior attempts to improve the external quantum efficiency of OLEDs by increasing the number of light beams that strike the substrate/air interface at an angle less than the critical angle.
For example Lai et al. (CLEO Conference Proceedings, Pacific Rim 99, WL6, pages 246-47 (1999)) suggests texturing the bottom light emitting surface of a glass substrate (i.e., the surface distal from the OLED device). The textured surface enables more light rays from the organic light emitting layer to strike the substrate/air interface at an angle smaller than the critical angle, thus allowing more light rays to escape from the substrate.
Furthermore, Madigan et al., 76 Appl. Phys. Lett. 13, 1650 (2000), incorporated herein by reference, have suggested placing a glass, silicone or epoxy lens array over a glass or polycarbonate substrate to reduce critical angle loss. Since the lens array has the same or similar index of refraction as the substrate, the loss at the substrate/array interface is minimized. However, the index of refraction of the glass and polycarbonate substrates (n=1.51 and 1.59, respectively), is significantly lower than the index of refraction of the adjacent indium tin oxide (ITO) anode layer (n≈1.8). Therefore, the OLED of Madigan et al. suffers from a large loss at the ITO/substrate interface due to the large index of refraction mismatch between these materials.
Therefore, despite the efforts, the external quantum efficiency of OLEDs, such as those of Lai et al. and Madigan et al., is still lower than desirable. The present invention is directed to overcoming or at least reducing the problem set forth above.